In laser material processing systems in which radiation from a laser such as a solid-state-laser or a fiber-laser is transported to material processed via an optical fiber (transport-fiber) it is usual practice to optimize the laser radiation beam quality delivered from the optical fiber. A common measure of beam quality delivered from the transport fiber is the beam-parameter product (BPP). The BPP is the product of the beam-waist (smallest beam-radius) and the divergence half-angle. The smaller the BPP, the better is the beam-quality. The BPP can be thought of as a measure of the tightness with which the beam can be focused.
For a fiber delivered beam the beam-waist is essentially at the output-end of the transport fiber and is dependent inter alia, on the core diameter of the fiber. The divergence half-angle is dependent, inter alia, on the numerical aperture of the fiber, and the NA of a focused beam launched into the input-end of the fiber.
Typically in a commercial fiber-coupled laser of a particular model or series, the delivered BPP will be fixed at some optimum value. That optimum value, however, may not be the best for a particular customer's application. There is a need for a simple means for tailoring the BPP of a fiber coupled laser, if needs be, to fit a particular application.